Pinchasik Bat-El, Steinkühler Jan, Wuytens Pieter, Skirtach Andre G, Fratzl Peter, Möhwald Helmuth
Department of Biomaterials, ‡Department of Theory and Bio-systems, and ⊥Emeritus Group of Interfaces, Max-Planck Institute of Colloids and Interfaces , Golm/Potsdam, D14476 Germany.
Photonics Research Group, IMEC, INTEC Department and ∥Department of Molecular Biotechnology and NB-Photonics, Ghent University , 9000 Ghent, Belgium.
Langmuir. 2015 Dec 29;31(51):13734-42. doi: 10.1021/acs.langmuir.5b03821. Epub 2015 Dec 14.
The controlled wetting and dewetting of surfaces is a primary mechanism used by beetles in nature, such as the ladybird and the leaf beetle for underwater locomotion.1 Their adhesion to surfaces underwater is enabled through the attachment of bubbles trapped in their setae-covered legs. Locomotion, however, is performed by applying mechanical forces in order to move, attach, and detach the bubbles in a controlled manner. Under synthetic conditions, however, when a bubble is bound to a surface, it is nearly impossible to maneuver without the use of external stimuli. Thus, actuated wetting and dewetting of surfaces remain challenges. Here, electrowetting-on-dielectric (EWOD) is used for the manipulation of bubble-particle complexes on unpatterned surfaces. Bubbles nucleate on catalytic Janus disks adjacent to a hydrophobic surface. By changing the wettability of the surface through electrowetting, the bubbles show a variety of reactions, depending on the shape and periodicity of the electrical signal. Time-resolved (μs) imaging of bubble radial oscillations reveals possible mechanisms for the lateral mobility of bubbles on a surface under electrowetting: bubble instability is induced when electric pulses are carefully adjusted. This instability is used to control the surface-bound bubble locomotion and is described in terms of the change in surface energy. It is shown that a deterministic force applied normal can lead to a random walk of micrometer-sized bubbles by exploiting the phenomenon of contact angle hysteresis. Finally, bubble use in nature for underwater locomotion and the actuated bubble locomotion presented in this study are compared.
表面的可控润湿和去湿是自然界中甲虫所采用的一种主要机制，例如瓢虫和叶甲虫在水下移动时就会用到。它们在水下对表面的附着是通过附着在其被刚毛覆盖的腿部所捕获的气泡来实现的。然而，移动是通过施加机械力来进行的，以便以可控的方式移动、附着和分离气泡。然而，在合成条件下，当一个气泡附着在表面时，如果不使用外部刺激，几乎不可能对其进行操控。因此，表面的驱动润湿和去湿仍然是挑战。在此，介电电泳（EWOD）被用于操控无图案表面上的气泡 - 颗粒复合物。气泡在与疏水表面相邻的催化性Janus盘上成核。通过介电电泳改变表面的润湿性，气泡会根据电信号的形状和周期性表现出各种反应。对气泡径向振荡的时间分辨（微秒级）成像揭示了在介电电泳作用下气泡在表面横向移动的可能机制：当仔细调整电脉冲时会引发气泡不稳定性。这种不稳定性被用于控制表面附着气泡的移动，并根据表面能的变化来描述。结果表明，通过利用接触角滞后现象，垂直施加的确定性力可导致微米级气泡的随机游动。最后，对自然界中气泡用于水下移动与本研究中所展示的驱动气泡移动进行了比较。
